Back when Google updated the AdSense terms of service in January, they began allowing publishers to place AdSense ads on the same page with Yahoo Publisher Network ads and other contextual ads.
I don't think we made strong emphasis of that change at that time. But a WebmasterWorld thread brings that topic back to life.
AdSenseAdvisor has confirmed this to be true:
Yes, I can confirm that this was a change we made in January. Publishers are permitted to run Google ads on the same pages as other contextually targeted ads as long as the formatting and implementation are sufficiently different so the other ads will not be mistaken for Google ads, or vice versa.
The program policies clearly state:
Competitive Ads and Services
In order to prevent user confusion, we do not permit Google ads or search boxes to be published on websites that also contain other ads or services formatted to use the same layout and colors as the Google ads or search boxes on that site. Although you may sell ads directly on your site, it is your responsibility to ensure these ads cannot be confused with Google ads.
Now, practically speaking, you still cannot put your AdSense ads on the same page as your YPN ads. Why? Simply because Yahoo does not allow other contextual ads on the same page as the YPN ads. It violates their terms of service and you can be banned.
Forum discussion at WebmasterWorld.